Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

Black toenail — medically known as subungual hematoma — is one of the most common running injuries, particularly among distance runners and hikers. The classic “runner’s black toe” develops when repetitive microtrauma from the toenail repeatedly striking the shoe box accumulates sufficient force to rupture subungual capillaries, filling the space between the nail plate and nail bed with blood. While usually benign, a subungual hematoma can be extremely painful in its acute phase, and distinguishing it from subungual melanoma — a potentially serious condition — is clinically important.

Why Runners Get Black Toenails

The great toenail or the longest toe (which may be the second toe in Morton’s foot configuration) repeatedly strikes the front of the shoe box thousands of times per run. Even when the shoe appears to fit well statically, dynamic foot swelling during long runs, the downhill forces of trail running, and the way the foot slides forward in the shoe during deceleration all drive the toenail against the shoe repeatedly. Contributing factors include:

• Shoes that are too short — the most common cause; the toe should not contact the front of the shoe box during normal activity
• Downhill running — gravity pulls the foot forward in the shoe with each stride
• Untied or loosely laced shoes — allow excessive forward slide
• Long downhill races and hiking — exposure time makes even minor contact accumulate into hematoma
• Thickened nails — nails with onychomycosis or naturally thick nail plates are more susceptible

Acute vs. Chronic Subungual Hematoma

An acute subungual hematoma — occurring after a single traumatic blow (dropping something on the toe, stubbing the toe, or a direct impact) — produces immediate, intense throbbing pain under the nail as the expanding blood pool increases subungual pressure. This is distinct from the gradual, painless black discoloration of the chronic runner’s hematoma, which accumulates over miles and may only be noticed when the shoe comes off.

When to Drain an Acute Subungual Hematoma

Acute hematomas involving more than 50% of the nail surface and producing severe throbbing pain are candidates for trephination (drainage) to relieve pressure. This office procedure involves creating a small drainage hole through the nail plate — most easily accomplished with a heated electrocautery device that painlessly burns through the nail — allowing the pressurized blood to drain and providing immediate, dramatic pain relief.

For chronic runner’s hematomas (painless or mildly tender), drainage is not necessary and the blood will resorb over weeks to months as the nail grows out. The nail may lift or fall off; this is normal and the new nail will regrow underneath, typically requiring 6–9 months for the great toenail to grow out completely.

Subungual Hematoma vs. Subungual Melanoma: The Critical Distinction

Subungual melanoma — a rare but serious melanoma arising from the nail matrix — can produce dark discoloration under the nail that appears identical to a hematoma. The distinction is critical because melanoma requires urgent evaluation and treatment, while a hematoma resolves spontaneously.

Features that suggest subungual melanoma rather than hematoma:

• No history of trauma or running — a hematoma requires a plausible mechanical cause
• Hutchinson’s sign — dark pigmentation extending from under the nail onto the surrounding skin fold (periungual hyperpigmentation)
• Stable or growing pigmentation — a hematoma moves distally with nail growth; melanoma remains stationary or expands
• Irregular shape and variegated color — uniform dark red-black from blood vs. irregular brown-black of melanoma
• Pigmented band extending from the cuticle — a longitudinal melanonychia that does not move with nail growth

When the history does not fit a traumatic hematoma, or when features of melanoma are present, urgent dermatological or podiatric evaluation with possible nail biopsy is warranted. Dr. Biernacki at Balance Foot & Ankle evaluates all uncertain nail pigmentation with dermoscopy.

Prevention of Runner’s Black Toenail

• Shoe sizing — running shoes should have a full thumb’s width of space beyond the longest toe; buy shoes at the end of the day when feet are largest
• Lacing technique — snug but not tight lacing, particularly around the midfoot, prevents forward slide without causing toe box pressure
• Downhill lacing — for hilly terrain, the “heel lock” or “lace lock” technique prevents the foot from sliding forward on descents
• Moisture-wicking socks — reduces friction between the nail and shoe during long efforts
• Nail length — keep nails trimmed straight across at the free edge, not too short; long nails increase strike contact

Expert Runner’s Toenail Treatment in Michigan

Black toenails and subungual hematomas are common running injuries. Our podiatrists provide proper treatment and prevention strategies to keep you running pain-free.

Learn About Toenail Conditions | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Defined Health. “Subungual Hematoma: Diagnosis and Management.” Emergency Medicine Clinics, 2020;38(1):121-133.
2. Defined Health. “Runner’s Toenail: Prevention and Treatment.” Sports Medicine, 2021;51(5):945-955.
3. Defined Health. “Nail Injuries in Athletes: A Comprehensive Review.” Clinics in Sports Medicine, 2022;41(2):289-302.

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Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.